The present invention relates to a coordinate input device capable of detecting the coordinates of a point to which pressure is applied by a writing pen or a finger, and to an input display apparatus including the particular coordinate input device.
Known coordinate input devices of this kind include, for example, a pressure sensitive type device in which the coordinates are detected on the basis of pressure applied by a writing pen or a finger, and an electrostatic coupling type or electromagnetic coupling type device in which an electrostatic coupling or electromagnetic induction between a writing pen and an electrode is utilized. These coordinate input devices are used in combination with display devices such as a liquid crystal display device to form input display apparatuses capable of input and display of information.
Known input display apparatuses include a type that a transparent coordinate input device is arranged on the front surface of a display device, and another type that a transparent coordinate input device is arranged on the back surface of a display device. The coordinate input device used in the input display apparatus of the former type includes the pressure sensitive type device, electrostatic coupling type device and electromagnetic coupling type device. On the other hand, the electromagnetic coupling type coordinate input device is used in the input display apparatus of the latter device.
In manufacturing the input display apparatus having an integral structure of a coordinate input device and a display device, it is necessary to employ the step of bonding the coordinate input device to the display device. In the bonding step, it is difficult to align accurately the coordinate input device with the display device, with the result that a positional deviation tends to take place between the display device and the coordinate input device. For eliminating the positional deviation, it is necessary to correct the electrical circuit for each input display device product, leading to an increased manufacturing cost of the input display apparatus.
It should also be noted that, in bonding the coordinate input device directly to the display device, an interference band is generated by uneven gaps between the two devices, leading to a marked deterioration in the display performance of the input display apparatus. To overcome this difficulty, it is necessary to provide a gap of about 1 mm between the two devices. As a result, the display surface of the display device is made apart from the input surface of the coordinate input device by a distance equal to sum of the thickness of the coordinate input device and the gap provided between the coordinate input device and the display device, giving rise to generation of parallax between the display position and the coordinate input position depending on the viewing direction of the user. In addition, the presence of the gap between the two devices makes it difficult to decrease the thickness of the module consisting of the coordinate input device and the display device.
Further, in the input display apparatus of the former type, the reduction of transmittance caused by the coordinate input device remains to be a serious problem to be solved. Particularly, in an apparatus using a reflection type liquid crystal display device, the ambient light passes through the coordinate input device twice, leading to a further reduction of the transmittance. For example, a pressure sensitive type coordinate input device comprises in general two flexible substrates each having a transparent electrode formed thereon. These two flexible substrates are arranged to face each other with spacers disposed therebetween so as to provide a predetermined free space between the two. In an input display apparatus having the particular coordinate input device of the pressure sensitive type arranged on the front surface of a liquid crystal display device, both a counter electrode of the liquid crystal display device and two transparent electrodes of the coordinate input device are arranged in the optical path. In addition, a gap is defined between the coordinate input device and the liquid crystal display device. It follows that the light transmittance is markedly lowered, making it difficult to achieve display of images of a high brightness.
As described above, in an integral structure having a coordinate input device and a display device, a positional deviation takes place in the bonding step of these devices, leading to reduction in the input accuracy. Also, a positional deviation between these devices in the thickness direction brings about parallax between the display position and the coordinate input position depending on the viewing angle. Further, the coordinate input device, which is of a laminate structure comprising a plurality of films, is low in its light transmittance. It follows that the quality of the image displayed by the input display apparatus utilizing the coordinate input device is lowered, particularly where a reflection type liquid crystal display device is used in combination with the coordinate input device.